Solid oxide fuel cell system

ABSTRACT

The present invention is a solid oxide fuel cell system for generating variable power in response to power demand, having: a fuel cell module; a fuel supply device; a power demand detection device; a controller for controlling the amount of fuel supplied by the fuel supply device based on the power demand, and for setting an extractable current value, being the maximum extractable current value; an inverter for extracting current from the fuel cell module within a range not exceeding the extractable current value; and an extractable current detection device for detecting actual extracted current extracted from the fuel cell module; whereby if certain increase-limiting condition is matched, then even when power demand is rising, the controller maintains the extractable current value at a certain value, or lowers the extractable current value, and does not increase that extractable current value.

TECHNICAL FIELD

The present invention pertains to a solid oxide fuel cell system, andmore particularly to a solid oxide fuel cell system for generatingvariable electrical power in response to power demand.

BACKGROUND ART

In recent years, various solid oxide fuel devices comprising fuel cellscapable of obtaining electrical power by generating electricity usingfuel (hydrogen gas) and air, as well as auxiliary equipment for runningsuch fuel cells, have been proposed as next-generation energy sources.

Japanese Unexamined Patent Application H.7-307163 (Patent Document 1)sets forth a fuel cell device. In this fuel cell device, the electricalpower generated is varied in response to load.

Here, referring to FIG. 15, we explain a power supply system utilizing afuel cell. FIG. 15 shows an example of a conventional system forsupplying electrical power to a residence using a fuel cell. In thesystem, electrical power consumed by a residence 200 is supplied by fuelcell 202 and grid power 204. Normally the maximum consumed powerconsumed by residences is larger than the maximum rated powergeneratable by fuel cell 202, therefore even in a residence 200utilizing a fuel cell 202, that insufficiency is made up for by gridpower 204, and electrical power is supplied to the residence from fuelcell 202 and grid power 204. Furthermore, even in situations where thegenerating capacity of fuel cell 202 is below the maximum rated powerfor a residence, a portion of the electrical power consumption ofresidence 200 is normally supplied from the grid power 204 in order toprevent reverse current flow of generated power to current power 204.

Grid power 204 is power fed from a transmission line to an electricaldistribution panel inside a residence, and is purchased power. In otherwords, the total of the electrical power generated by fuel cell 202 andgrid power 204 corresponds to the power consumed by residence 200. Fuelcell 202 obtains a monitor signal from power demand detector 206 of theelectrical power purchased by residence 200, and based on this it variesthe power generated by fuel cell 202. I.e., fuel cell 202 determines abase current Ii expressing the current which fuel cell 202 shouldproduce based on the monitor signal obtained from power demand detector206, and controls the amount of fuel, etc. supplied to fuel cell module208 to enable production of this base current Ii. Also, base current Iiis set at or below a value corresponding to the maximum rated power offuel cell 202, regardless of the power consumed by residence 200.

The fuel cell module 208 built into fuel cell 202 generally has anextremely slow response, making it difficult to change generated powerto follow changes in power consumed by residence 200. Therefore the basecurrent Ii signal which instructs an electrical generation amount tofuel cell module 208 is determined by applying a filter 210 whichperforms integration or the like on the monitor signal, so that itchanges extremely gradually compared to the change in power consumption.

Fuel cell 202 supplies fuel cell module 208 with an amount of fuelproportional to base current Ii so that fuel cell module 208 has thecapacity to produce the base current Ii. At the same time, inverter 212extracts a DC extracted current Ic from fuel cell module 208 andconverts this to AC and supplies it to residence 200. The actualextracted current Ic which inverter 212 extracts from fuel cell module208 is at all times set at or below the value of base current Ii, anddoes not exceed the generating capacity of fuel cell module 208. If acurrent equal to or greater than the generating capacity correspondingto the fuel supply amount, etc. determined based on base current Ii isextracted from fuel cell module 208, then there is a risk that fueldepletion in fuel cells within the fuel cell module 208 will occur,dramatically shortening the life span of the fuel cells and damaging thefuel cells.

At the same time, because of sharp fluctuations in the power consumed byresidence 200, when consumed power suddenly drops, the residence 200power consumption drops further than the power corresponding to basecurrent Ii, which is slowly varied.

In the fuel cell apparatus set forth in Japanese Unexamined PatentApplication H07-307163, when the current value is increased to adapt tothis type of delay in fuel cell module 208, the set current value isupdated through a delay setting instrument, and problems such as fueldepletion are prevented by delaying the increase in the set currentvalue. Also, in this fuel cell apparatus, when updating the set currentvalue, changes are always made in increments of subtracted currentvalues or added current values, therefore the rate of change at whichthe said current value is changed is always fixed.

Japanese Unexamined Patent Application H7-307163 SUMMARY OF THEINVENTION Problems the Invention Seeks to Resolve

In the fuel cell apparatus set forth in Japanese Unexamined PatentApplication H7-307163, increases in power demand (power consumption) areaccompanied by an increase in fuel gas, followed by an increase incurrent extracted from the fuel cell after a fixed time delay. Thereforewhen power demand increases, the current extracted from the fuel cell isalways increased while waiting for a fixed time delay.

However, in the fuel cell apparatus set forth in Japanese UnexaminedPatent Application H7-307163, current extracted from the fuel cell issimply made to follow power demand at a delay, so although it ispossible to prevent fuel depletion and the like, the problem arises thatfuel cell module operation can become unstable, etc., inducing a drop ingenerating efficiency.

Therefore the object of the present invention is to provide a solidoxide fuel cell system capable of enabling stable fuel cell moduleoperation and increasing generating efficiency while reliably avoidingdamage to the fuel cell module by fuel depletion or the like.

Means for Resolving Problems

To solve the above-described problems, the present invention is a solidoxide fuel cell system for generating variable power in response topower demand, comprising: a fuel cell module that generates electricityusing supplied fuel; a fuel supply device that supplies fuel to the fuelcell module; a power demand detection device that detects power demand;a controller that controls the amount of fuel supplied by the fuelsupply device based on the power demand detected by the power demanddetection device, and that sets an extractable current value, being themaximum current value which can be extracted from the fuel cell modulein accordance with the condition of the fuel cell module; an inverterthat converts current from the fuel cell module to alternating currentwithin a range not exceeding the extractable current value; and anextractable current detection device that detects the actual extractedcurrent actually extracted from the fuel cell module at the inverter;wherein if predetermined increase-limiting condition is matched, theneven when power demand is rising, the controller maintains the constantextractable current value, or lowers the extractable current value, anddoes not increase the extractable current value.

In the invention thus constituted, the controller controls the amount offuel supplied by a fuel supply device based on power demand detected bya power demand detection device to provide fuel to a fuel cell module.Furthermore, the controller sets an extractable current value, which isthe maximum current value which can be extracted from the fuel cellmodule according to the state of the fuel cell module. The inverterextracts power from the fuel cell module in a range such that currentproportional to power demand does not exceed the extractable currentvalue. If certain increase-limiting condition is matched, then even whenpower demand is rising, the controller maintains the extractable currentvalue at a certain value, or lowers the extractable current value, anddoes not increase that extractable current value.

In general, the inverter is controlled with high responsiveness so thatit can extract current needed from the fuel cell module in response tosuddenly changing power demands. On the other hand, if the fuel supplyamount supplied to the fuel cell module is suddenly changed, electricalgeneration by the fuel cell module can become unstable, preventingachievement of highly responsive control. In addition, multiple factorssuch as temperature affect the generating capacity of the fuel cellmodule, and increasing the supply of fuel does not necessarily lead toan increase in fuel cell generating capacity. Therefore if the state ofthe fuel cell module is ignored and the supply of fuel is simplyincreased while at the same time extracted current is increased, anexcessive load can be imposed on the fuel cell module, hastening thedegradation of the fuel cell module.

According to the invention, under circumstances matching certainincrease-limiting condition the controller does not increase theextracted current value, but rather maintains the extracted current at afixed value, or reduces the extracted current value, even when powerdemand is rising, therefore damage to fuel cell modules by fueldepletion or the like can be reliably avoided, and the fuel cell modulestably operated. Also, maintaining or reducing extracted current evenwhen power demand is rising enables rapid recovery from improperconditions or poor efficiency conditions of electrical generation by thefuel cell module, and compared to immediately increasing the extractedcurrent value, this method suppresses effects on the fuel cell modulewhile simultaneously actually increasing generating efficiency.

In the present invention the controller is preferably constituted tojudge the increase-limiting condition based on certain parameter, suchthat even when power demand is rising, if predetermined parameterexceeds certain threshold, extractable power is maintained at a fixedvalue, and if the excess amount over predetermined threshold increasesstill further, extractable power is reduced.

In the invention thus constituted, increase-limiting condition is judgedbased on certain parameter value, and when that parameter value isinappropriate for increasing the extractable current value, theextractable current value is maintained at a fixed value. In thiscondition, if a parameter value degrades even further, the extractablecurrent value is lowered.

In the invention thus constituted, when certain parameter value isinappropriate for increasing the extractable current value, extractablecurrent is maintained at a fixed value, so no new load is placed on thefuel cell module, and it can be discerned whether the state of aslow-response fuel cell module will or will not recover. If the state ofthe fuel cell module degrades still further, extractable current can bereduced to actively lighten the load on the fuel cell module.

In the present invention the increase-limiting condition preferablyincludes a current maintenance condition for maintaining extractablecurrents at a fixed value, and a current reducing condition for reducingextractable current, and the controller applies the current reducingcondition with priority over the current maintenance condition.

In the invention thus constituted, priority is given to the currentreducing condition over the current maintenance condition, so if a fuelcell module might be damaged by the continuation of a given load on thefuel cell module, that load can be quickly lightened and damage to thefuel cell module reliably prevented.

In the invention thus constituted, the controller is preferably providedwith multiple current maintenance conditions and current reducingconditions, respectively, and the extractable current value is increasedwhen none of multiple current maintenance conditions is met, and isreduced when even one of the multiple current reducing conditions ismet.

In the invention thus constituted, an increase in extractable currentposing a large load on the fuel cell module is only executed when thereis no match with any of the current maintenance conditions, thereforewhen the state of the fuel cell module is not good enough to increasethe extractable current value, that extractable current value ismaintained at a fixed value. If the extractable current value matcheseven one of the current reducing conditions, it is immediately reduced,and damage to the fuel cell module can be reliably prevented.

In the present invention the controller preferably changes theextractable current value so that the rate of change at which theextractable current value is reduced varies according to which of themultiple current reducing conditions was met.

In the invention thus constituted, in conditions where the load on thefuel cell module must be rapidly reduced, the extractable current valuecan be rapidly reduced to quickly reduce load, and in conditions where arapid load reduction is not required, the time needed for recovery ofthe necessary extractable current value can be shortened by graduallyreducing the extractable current value.

In the present invention, increase-limiting condition is preferablyjudged based multiple parameters selected from among fuel cell moduletemperature, fuel cell module output voltage, actual extracted current,and extractable current value.

In the invention thus constituted, the electrical generating capacity ofthe fuel cell module can be quickly restored while suppressingdegradation of the fuel cell module by parallel monitoring of multipleparameters with differing effects on fuel cell module generatingcapacity.

In the invention thus constituted, the temperature of the fuel cellmodule is preferably judged based on a lower limit threshold value andan upper limit threshold value; if the temperature of the fuel cellmodule drops below the lower limit threshold value, the extractablecurrent value is maintained at a fixed value; if the fuel cell moduletemperature drops still further, the extractable current value islowered; if the fuel cell module temperature rises past the upper limitthreshold value, the extractable current value is reduced.

In the invention thus constituted, on the fuel cell module temperaturereduction side the time needed until the requisite extractable currentvalue is restored can be shortened by maintaining the extractablecurrent value and waiting for the temperature to recover, and byreducing the extractable current value if there is a further drop intemperature. On the fuel cell module temperature increase side, damageto the fuel cell module can be prevented by immediately reducing theextractable current value.

Effect of the Invention

Using the solid oxide fuel cell system of the present invention, stableoperation of the fuel cell module is enabled while reliably avoidingdamage to the fuel cell module by fuel depletion or the like, andelectrical generating efficiency can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: An overview diagram showing a solid oxide fuel cell systemaccording to an embodiment of the present invention.

FIG. 2: A front elevation cross section showing the fuel cell module ina fuel cell system according to an embodiment of the present invention.

FIG. 3: A cross section along line in FIG. 2.

FIG. 4: A partial cross section showing an individual fuel cell unit ina fuel cell system according to an embodiment of the present invention.

FIG. 5: A perspective view showing a fuel cell stack in a fuel cellsystem according to an embodiment of the present invention.

FIG. 6: A block diagram showing a fuel cell assembly according to anembodiment of the present invention.

FIG. 7: A timing chart showing the operation at startup of a fuel cellassembly according to an embodiment of the present invention.

FIG. 8: A timing chart showing the operation of a fuel cell systemaccording to an embodiment of the present invention when stopped.

FIG. 9: A control table of the startup processing procedure in a fuelcell system according to an embodiment of the present invention.

FIG. 10: A flow chart showing control executed by a control section.

FIG. 11: A flow chart showing control executed by a control section.

FIG. 12: A timing chart showing the operation of a fuel cell systemaccording to an embodiment of the present invention.

FIG. 13: A timing chart showing the operation of a fuel cell systemaccording to an embodiment of the present invention.

FIG. 14: A timing chart showing the operation of a fuel cell systemaccording to an embodiment of the present invention.

FIG. 15: An example of a conventional system for supplying electricalpower to a residence using a fuel cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Next, referring to the attached drawings, we discuss a solid oxide fuelcell system (SOFC) according to an embodiment of the present invention.

FIG. 1 is an overview schematic showing an solid oxide fuel cell system(SOFC) according to an embodiment of the present invention. As shown inFIG. 1, solid oxide fuel cell system (SOFC) 1 according to an embodimentof the present invention comprises a fuel cell module 2 and an auxiliaryunit 4.

Fuel cell module 2 comprises a housing 6; inside this housing 6, asealed space 8 is formed, mediated by thermal insulation (not shown;thermal insulation is not an essential structure, and be can beomitted). Note that it is acceptable not to provide thermal insulation.Fuel cell assembly 12, which performs an electricity generating reactionusing fuel gas and oxidant (air), is disposed on generating chamber 10,under this sealed space 8. This fuel cell assembly 12 comprises ten fuelcell stacks 14 (see FIG. 5); fuel cell stacks 14 comprise 16 individualfuel cell units 16 (see FIG. 4). Thus fuel cell assembly 12 has 160individual fuel cell units 16, and all of these individual fuel cellunits 16 are connected in series.

A combustion chamber 18 is formed above the aforementioned generatingchamber 10 in fuel cell module 2 sealed space 8; residual fuel gas andresidual oxidizer (air) not used in the electricity generating reactionare burned in this combustion chamber 18, producing exhaust gas.

Reformer 20 for reforming fuel gas is disposed above this combustionchamber 18; reformer 20 is heated to a temperature at which thereforming reaction can occur by the combustion heat of the residual gas.Furthermore, air heat exchanger 22 for receiving heat from reformer 20and heating air to suppress temperature drops in reformer 20 is disposedabove reformer 20.

Next, auxiliary unit 4 comprises pure water tank 26, which stores waterfrom water supply source 24 and uses a filter to produce pure water, andwater flow volume regulator unit 28 (a motor-driven “water pump” or thelike), which regulates the flow volume of water supplied from thisholding tank. Auxiliary unit 4 comprises gas shutoff valve 32 forshutting off fuel gas such as municipal gas supplied from fuel supplysource 30, desulfurizer 32 for removing sulfur from fuel gas, and fuelflow regulator unit 38 (a motor-driven “water pump” or the like) forregulating the flow volume of fuel gas. Furthermore, auxiliary unit 4comprises: an electromagnetic valve 42 for shutting off air, which isthe oxidant supplied from air supply source 40, reform air flowregulator unit 44 and generating air flow regulator unit 45 (amotor-driven “water pump” or the like), which regulate the flow volumeair, first heater 46 for heating reforming air supplied to reformer 20,and second heater 48 for heating air supplied to the electricalgenerating chamber. This first heater 46 and second heater 48 areprovided in order to efficiently raise the temperature at startup, butmay also be omitted.

Next, a hot water production device 50, supplied with exhaust gas, isconnected to fuel cell module 2. Tap water is supplied from water supplysource 24 to this hot water production device 50; this tap water becomeshot water using the heat of the exhaust gas, and is supplied to anexternal hot water holding tank, not shown.

A control box 52 for controlling the amount of fuel gas supplied, etc.is connected to the fuel cell module 2.

Furthermore, an inverter 54 serving as an electrical power extractionunit (power conversion unit) for supplying electrical power generated bythe fuel cell module to the outside is connected to fuel cell module 2.

Next, using FIGS. 2 and 3, we explain the internal structure of a solidoxide fuel cell system (SOFC) according to the present embodiment of theinvention. FIG. 2 is a side view cross section showing the fuel cellmodule in a solid oxide fuel cell system (SOFC) according to anembodiment of the invention; FIG. 3 is a cross section along lineIII-III of FIG. 2.

As shown in FIGS. 2 and 3, starting from the bottom in the sealed space8 within the fuel cell module 2 housing 6, a fuel cell assembly 12, areformer 20, and an air heat exchanger 22 are arranged in sequence, asdescribed above.

A pure water guide pipe 60 for introducing pure water into the upstreamend of reformer 20, and a reform gas guide pipe 62 for introducing fuelgas and reforming air to be reformed, are attached to reformer 20; avaporizing section 20 a and a reforming section 20 b are formed insequence starting from the upstream side within reformer 20, and thesereforming sections 20 a and 20 b are filled with reforming catalyst.Fuel gas and air, blended with steam (pure water) introduced intoreformer 20, is reformed using the reforming catalyst with whichreformer 20 is filled. Reforming catalysts in which nickel is applied tothe surface of aluminum spheres, or ruthenium is imparted to the surfaceof aluminum spheres, are used as appropriate.

A fuel gas supply line 64 is connected to the downstream end of reformer20; this fuel gas supply line 64 extends downward, then further extendshorizontally within a manifold formed under fuel cell assembly 12.Multiple fuel supply holes 64 b are formed on the bottom surface of thehorizontal portion 64 a of fuel gas supply line 64; reformed fuel gas issupplied into manifold 66 from these fuel supply holes 64 b.

A lower support plate 68 provided with through holes for supporting theabove-described fuel cell stack 14 is attached at the top of manifold66, and fuel gas in manifold 66 is supplied into fuel cell units 16.

Next, air heat exchanger 22 is provided above reformer 20. This air heatexchanger 22 comprises an air concentration chamber 70 on the upstreamside and two air distribution chambers 72 on the downstream side; thisair concentration chamber 70 and distribution chambers 72 are connectedusing six air flow conduits 74. Here, as shown in FIG. 3, three air flowconduits 74 form a set (74 a, 74 b, 74 c, 74 d, 74 e, 74 f); air in airconcentration chamber 70 flows from each set of air flow conduits 74 tothe respective air distribution chambers 72.

Air flowing in the six air flow conduits 74 of the air heat exchanger 22is pre-heated by exhaust gas rising after combustion in combustionchamber 18.

Air guide pipes 76 are connected to each of the respective airdistribution chambers 72; these air guide pipes 76 extend downward,communicating at the bottom end side with the lower space in generatingchamber 10, and introducing preheated air into generating chamber 10.

Next, an exhaust gas chamber 78 is formed below manifold 66. As shown inFIG. 3, a vertically extending exhaust gas conduit 80 is formed on theinside of front surface 6 a and rear surface 6 b, which are faces in thelongitudinal direction of housing 6; the top end of exhaust gas chamberconduit 80 communicates with the space where air heat exchanger 22 isdisposed, and the bottom end communicates with exhaust gas chamber 78.An exhaust gas discharge pipe 82 is connected at approximately thecenter of the bottom surface of the exhaust gas chamber 78; thedownstream end of this exhaust gas discharge pipe 82 is connected to theabove-described hot water production device 50 shown in FIG. 1.

As shown in FIG. 2, an ignition device 83 for starting the combustion offuel gas and air is disposed on combustion chamber 18.

Next, referring to FIG. 4, we explain fuel cell units 16. FIG. 4 is apartial cross section showing the fuel cell units of the solid oxidefuel cell system (SOFC) according to an embodiment of the invention.

As shown in FIG. 4, fuel cell units 16 are furnished with a fuel cell 84and internal electrode terminals 86, respectively connected to theterminals at the top and bottom of fuel cell 84.

Fuel cell 84 is a tubular structure extending in the vertical direction,furnished with a cylindrical internal electrode layer 90, on the insideof which are formed a fuel gas flow path 88, a cylindrical externalelectrode layer 92, and an electrolyte layer 94 between internalelectrode layer 90 and external electrode layer 92. This internalelectrode layer 90 is a fuel electrode through which fuel gas passes,and has a (−) polarity, while the external electrode layer 92 is anair-contacting electrode with a (+) polarity.

The internal electrode terminals 86 attached at the top and bottom endsof fuel cell units 16 have the same structure, therefore we herespecifically discuss internal electrode terminal 86 attached at the topend. The top portion 90 a of inside electrode layer 90 comprises anoutside perimeter surface 90 b and top end surface 90 c, exposed toelectrolyte layer 94 and outside electrode layer 92. Inside electrodeterminal 86 is connected to the outer perimeter surface of insideelectrode layer 90 through conductive seal material 96, and iselectrically connected to inside electrode layer 19 by direct contactwith the top end surface 90 c of inside electrode layer 90. A fuel gasflow path 98 communicating with inside electrode layer 90 fuel gas flowpath 88 is formed at the center portion of inside electrode terminal 86.

Inside electrode layer 90 is formed, for example, from at least one ofthe following: a mixture of Ni with zirconia doped with Ca or at leastone rare earth element selected from among Y, Sc, or the like; a mixtureof Ni with ceria doped with at least one element selected from amongrare earth elements; or a mixture of Ni with lanthanum gallate dopedwith at least one element selected from among Sr, Mg, Co, Fe, or Cu.

Electrolyte layer 94 is formed, for example, from at least one of thefollowing: zirconia doped with at least one type of rare earth elementselected from among Y, Sc, or the like; ceria doped with at least onetype of element selected from among rare earth elements; or lanthanumgallate doped with at least one element selected from Sr or Mg.

The outside electrode layer 92 is formed, for example, from at least oneof the following: lanthanum manganite doped with at least one elementselected from among Sr or Ca; lanthanum ferrite doped with at least oneelement selected from among Sr, Co, Ni, or Cu; lanthanum cobaltite dopedwith at least one element selected from among Sr, Fe, Ni, or Cu; silver,or the like.

Next, referring to FIG. 5, we explain fuel cell stack 14. FIG. 5 is aperspective view showing the fuel cell stack in a solid oxide fuel cellsystem (SOFC) according to an embodiment of the present invention.

As shown in FIG. 5, fuel cell stack 14 is furnished with 16 fuel cellunits 16; the top inside and bottom inside of these fuel cell units 16are respectively supported by a lower support plate 68 and upper supportplate 100. Through holes 68 a and 100 a, through which inside electrodeterminal 86 can penetrate, are provided on this lower support plate 68and outer support plate 100.

In addition, a collector 102 and an external terminal 104 are attachedto fuel cell units 16. This collector 102 is integrally formed by a fuelelectrode connecting portion 102 a, electrically connected to insideelectrode terminal 86 attached to inside electrode layer 90 serving asthe fuel electrode, and by an air electrode connecting portion 102 b,electrically connected to the entire external perimeter of outsideelectrode layer 92 serving as the air electrode. Air electrodeconnecting portion 102 b is formed of a plumb portion 102 c extendingvertically along the surface of outside electrode layer 92, and multiplehorizontal portions 102 d extending horizontally from this verticalportion 102 c along the surface of outside electrode layer 92. Fuelelectrode connecting portion 102 a extends in a straight line, in anupward or downward diagonal direction from the vertical portion 102 c ofair electrode connecting portion 102 b, toward inside electrodeterminals 86 positioned vertically on fuel cell units 16.

Furthermore, electrode terminals 86 at the top and bottom ends of thetwo fuel cell units 16 positioned at the end of fuel cell stack 14 (atthe front and back on the left side in FIG. 5) are respectivelyconnected to outside terminals 104. These external terminals 104 areconnected to external terminals 104 (not shown) at the ends of adjacentfuel cell stack 14, and as described above, all of the 160 fuel cellunits 16 are connected in series.

Next, referring to FIG. 6, we discuss the sensors attached to the solidoxide fuel cell system (SOFC) according to the present embodiment. FIG.6 is a block diagram showing a solid oxide fuel cell system (SOFC)according to an embodiment of the present invention.

As shown in FIG. 6, a solid oxide fuel cell system 1 comprises a controlunit 110; connected to this control section 110 are: an operating device112 provided with operating buttons such as “ON” or “OFF” for useroperation; a display device 114 for displaying various data such asgenerator output (watts); and a notification device 116 for issuingwarnings during abnormal states, etc. Note that this notification device116 may also be connected to a remote control center to inform thecontrol center of anomalies.

Next, signals from the various sensors described below are input tocontrol unit 110.

First, flammable gas detection sensor 120 is for detecting gas leaks,and is attached to fuel cell module 2 and auxiliary unit 4.

CO detection sensor 122 is for sensing whether CO in the exhaust gas,which is supposed to be exhausted to the outside via exhaust gas conduit80, etc., has leaked into the external housing (not shown) which coversfuel cell module 2 and auxiliary unit 4.

Water reservoir state detection sensor 124 is for sensing things such asthe temperature and amount of hot water in a hot water heater (notshown).

Electrical power state detection sensor 126 is for sensing current,voltage, etc. in inverter 54 and a distribution panel (not shown).

Generator air flow detection sensor 128 is for detecting the flow volumeof generating air supplied to generating chamber 10.

Reforming air flow volume sensor 130 is for detecting the volume ofreforming air flow supplied to reformer 20.

Fuel flow volume sensor 132 is for detecting the flow volume of fuel gassupplied to reformer 20.

Water flow volume sensor 134 is for detecting the flow volume of purewater supplied to reformer 20.

Water level sensor 136 is for detecting the water level in pure watertank 26.

Pressure sensor 138 is for detecting pressure on the upstream sideoutside reformer 20.

Exhaust temperature sensor 140 is for detecting the temperature ofexhaust gas flowing into hot water production device 50.

As shown in FIG. 3, generating chamber temperature sensor 142 isdisposed on the front surface side and rear surface side around fuelcell assembly 12, and has the purpose of detecting the temperature nearfuel cell stack 14 and estimating the temperature of fuel cell stack 14(i.e., of the fuel cell 84 itself).

Combustion chamber temperature sensor 144 is for detecting thetemperature in combustion chamber 18.

Exhaust gas chamber temperature sensor 146 is for detecting thetemperature of exhaust gases in exhaust gas chamber 78.

Reformer temperature sensor 148 is for detecting the temperature ofreformer 20; it calculates the reformer 20 temperature from the intakeand exit temperatures on reformer 20.

If a solid oxide fuel cell system (SOFC) is positioned outdoors, outsideair temperature sensor 150 detects the temperature of the outsideatmosphere. Sensors to detect atmospheric humidity and the like may alsobe provided.

Signals from these various sensors are sent to control unit 110; controlunit 110 sends control signals to water flow regulator unit 28, fuelflow regulator unit 38, reforming air flow regulator unit 44, andgenerating air flow regulator unit 45 based on data from the sensors,and controls the flow volumes in each of these units.

Next, referring to FIG. 7, we explain the operation of a solid oxidefuel cell system (SOFC) according to the present embodiment at the timeof start up. FIG. 7 is a timing chart showing the operation of a solidoxide fuel cell system (SOFC) according to an embodiment of the presentinvention at the time of start up.

At first, the operation starts in a no-load state, i.e., with thecircuit containing fuel cell module 2 in an open state, in order to warmup fuel cell module 2. At this point current does not flow in thecircuit, therefore fuel cell module 2 does not generate electricity.

First, reforming air is supplied from reforming air flow regulator unit44 through first heater 46 to reformer 20 in fuel cell module 2.Simultaneously, generating air is supplied from generating air flowregulator unit 45 through second heater 48 to the air heat exchanger 22on fuel cell module 2, and this generating air reaches generatingchamber 10 and combustion chamber 18.

Immediately thereafter, fuel gas is also supplied from fuel flowregulator unit 38, and fuel gas into which reforming air is blendedpasses through reformer 20, fuel cell stack 14, and fuel cell units 16to reach combustion chamber 18.

Next, ignition device 83 causes ignition, and fuel gas and air(reforming air and generating air) supplied to combustion chamber 18 arecombusted. This combustion of fuel gas and air produces exhaust gas;generating chamber 10 is warmed by this exhaust gas, and when theexhaust gas rises in the sealed space 8 of fuel cell module 2, the fuelgas, which includes reforming air in reformer 20, is warmed, as is alsothe generating air inside air heat exchanger 22.

At this point, fuel gas into which reforming air is blended is suppliedto reformer 20 by fuel flow regulator unit 38 and reforming air flowregulator unit 44, therefore the partial oxidation reforming reactionPOX given by Expression (1) proceeds. This partial oxidation reformingreaction POX is an exothermic reaction, and therefore has good startupcharacteristics. This elevated-temperature fuel gas is supplied fromfuel gas supply line 64 to the bottom of fuel cell stack 14, and by thismeans fuel cell stack 14 is heated from the bottom; combustion chamber18 is also heated by the combustion of fuel gas and air, so that fuelstack 14 is also heated from above, thereby enabling an essentiallyuniform rise in temperature along the vertical direction of fuel cellstack 14. Even though the partial oxidation reforming reaction POX isprogressing, the ongoing combustion reaction between fuel gas and air iscontinued in combustion chamber 18.

C_(m)H_(n) +xO₂ →aCO₂ +bCO+cH₂  (1)

After the partial oxidation reforming reaction starts, when reformertemperature sensor 148 senses that reformer 20 has reached apredetermined temperature (e.g., 600° C.), a pre-mixture of fuel gas andreforming air is supplied to reformer 20 by water flow regulator unit28, fuel flow regulator unit 38, and reforming air flow regulator unit44. At this point the auto-thermal reforming reaction ATR, which makesuse of both the aforementioned partial oxidation reforming reaction POXand the steam reforming reaction SR described below, proceeds inreformer 20. This auto-thermal reforming reaction ATR can be internallythermally balanced, therefore the reaction proceeds in a thermallyindependent fashion inside reformer 20. In other words, if oxygen (air)is abundant, heat emission by the partial oxidation reforming reactionPOX dominates, and if steam is abundant, the endothermic steam reformingreaction SR dominates. At this stage, the initial stage of startup haspassed and some degree of elevated temperature has been achieved withingenerating chamber 10, therefore even if the endothermic reaction isdominant, no major drop in temperature will be caused. Also, thecombustion reaction continues within combustion chamber 18 even whilethe auto-thermal reforming reaction ATR is proceeding.

When, after starting autothermal reforming reaction ATR given byExpression (2), reformer temperature sensor 146 senses that reformer 20has reached a predetermined temperature (e.g., 700° C.), the supply ofreforming air by reforming air flow regulator unit 44 is stopped and thesupply of steam by water flow regulator unit 28 is increased. A gascontaining no air and containing only fuel gas and steam is thussupplied to reformer 20, where the steam reforming reaction SR ofExpression (3) proceeds.

C_(m)H_(n)+XO₂ +yH₂O→aCO₂ +bCO+cH₂  (2)

C_(m)H_(n) +xH₂O→aCO₂ +bCO+cH₂  (3)

This steam reforming reaction SR is an endothermic reaction, thereforethe reaction proceeds while thermal balance is maintained with thecombustion heat from combustion chamber 18. At this stage, fuel cellmodule 2 is in the final stages of startup, therefore the temperaturehas risen to a sufficiently high level within generating chamber 10 sothat no major temperature drop is induced in generating chamber 10 eventhough an endothermic reaction is proceeding. Also, the combustionreaction continues to proceed in combustion chamber 18 even though thesteam reforming reaction SR is proceeding.

In this manner, after fuel cell module 2 has been ignited by ignitiondevice 83 the temperature inside generating chamber 10 gradually risesdue to the sequentially proceeding partial oxidation reforming reactionPOX, auto-thermal reforming reaction ATR, and steam reforming reactionSR. Next, when the temperatures of the interior of generating chamber 10and individual fuel cells 84 reach a predetermined generatingtemperature below the rated temperature at which fuel cell module 2 canbe stably operated, the circuit including fuel cell module 2 is closedand electrical generation by fuel cell module 2 begins, such thatcurrent flows in the circuit. Generation of electricity by fuel cellmodule 2 causes fuel cell 84 itself to emit heat, such that thetemperature of fuel cell 84 also rises. As a result, the ratedtemperature for operating fuel cell module 2, for example 600[° C.] to800[° C.], is reached.

In order to maintain the rated temperature thereafter, fuel gas and airare supplied in a quantity greater than the fuel gas and air consumed byindividual fuel cells 84, and combustion in combustion chamber 18 iscontinued. Note that during electrical generation, generation ofelectricity by the high reforming-efficiency steam reforming reaction SRproceeds.

Next, referring to FIG. 8, we discuss the operation when stopping thesolid oxide fuel cell system (SOFC) of the present embodiment. FIG. 8 isa timing chart showing what occurs upon stopping the operation of solidoxide fuel cell system (SOFC) of the present embodiment.

As shown in FIG. 8, when the operation of fuel cell module 2 is stopped,fuel flow regulator unit 38 and water flow regulator unit 28 are firstcontrolled to reduce the quantity of fuel gas and steam being suppliedto reformer 20.

When stopping the operation of fuel cell module 2, the amount ofgenerating air supplied by reforming air flow regulator unit 44 intofuel cell module 2 is being increased at the same time that the amountof fuel gas and steam being supplied to reformer 20 is being reduced;fuel cell assembly 12 and reformer 20 are air cooled to reduce theirtemperatures. Thereafter when the reformer 20 temperature has dropped toa predetermined temperature, for example 400[° C.], the supply of fuelgas and steam to the reformer 20 is stopped, and the reformer 20 steamreforming reaction SR is ended. Supply of generating air continues untilthe temperature in reformer 20 reaches a predetermined temperature, e.g.200° C., and when the predetermined temperature is reached, the supplyof generating air from generating air flow regulator unit 45 is stopped.

Thus in the present embodiment when operation of the fuel cell module 2is stopped, the steam reforming reaction SR by reformer 20 and coolingby generating air are used in combination, so that operation of the fuelcell module can be stopped relatively quickly.

Next, referring to FIG. 6, we explain the control of solid oxide fuelcell system 1 according to an embodiment of the invention.

First, as shown in FIG. 6, solid oxide fuel cell system 1 comprisescontrol section 110, which is a fuel cell controller, and invertercontrol section 111, which is an inverter controller.

Control section 110 comprises first power demand detection device 110 a,which detects power demand based on a power demand signal Ms input frompower demand detector 206 (FIG. 15). The total power demand consumed byfacilities like residence 200 (FIG. 15) is covered by grid powersupplied from commercial power sources and power supplied from solidoxide fuel cell system 1. If using a current transformer as power demanddetector 206, the grid current (purchased current) can be obtained as amonitor signal to serve as power demand monitor signal Ms, thereforepower demand can be obtained, together with grid power and electricalgeneration interconnected power, from the AC voltage at the outputterminal obtained from inverter 54, and from the electrical generationinterconnected output power. First power demand detection device 110 acan also be used, by indirectly obtaining that information from inverter54. In the present embodiment, it is the grid power of the total powerdemand that is input to control section 110 as the power demand monitorsignal Ms, but it is also possible for the control section to use totalpower demand as the power demand monitor signal.

Also, control section 110 is constituted to control water flow volumeregulator unit 28, fuel flow regulator unit 38, and reform air flowregulator unit 44, etc. based on power demand monitor signal Ms and thelike. Control section 110 sets the extractable current value Iinv basedon input signals from various sensors and on power demand monitor signalMs, and outputs this value to inverter control section 111.Specifically, control section 110 comprises a microprocessor, memory,programs for operating these, and so forth.

Inverter control section 111 comprises a second power demand detectiondevice 111 a, and detects power demand based on the power demand monitorsignal Ms input from power demand detector 206 (FIG. 15). When using acurrent transformer for power demand detector 20, the grid current(purchased current) is obtained as a monitor signal for use as powerdemand monitor signal Ms, therefore power demand is obtained togetherwith grid power and electrical generation interconnected power using anoutput terminal AC voltage from a voltage detection means provided on anoutput terminal, obtained from inverter 54, and the electricalgeneration interconnected output power from an output power detectionmeans on the output section. That information can also be conveyed tocontrol section 110. Inverter control section 111 controls inverter 54based on power demand monitor signal Ms and the extractable currentvalue Iinv input from control section 110, and actual extracted currentIc is extracted from fuel cell module 2 within a range not exceedingextractable current value Iinv. Specifically, inverter control section111 comprises a microprocessor, memory, programs for operating these,and the like.

Control section 110 comprises an extractable current setting means forsequentially setting extractable current value Iinv, being the maximumcurrent extractable from fuel cell module 2 at a given time in responseto the state of fuel cell module 2. Inverter control section 111controls inverter 54 independently of control section 110, extractsactual extracted current Ic in a range not exceeding the extractablecurrent value Iinv input from control section 110, and suppliesfacilities such as residence 200 (FIG. 15). Note that in the presentembodiment the control section 110 control cycle is 500 [msec], and theinverter control section 111 control cycle is 1 [msec] or less. Thuscontrol section 110 is operated at a control cycle necessary andsufficient to control a slow-response fuel cell module 2, and invertercontrol section 111 is operated at a short control cycle so that powercan be extracted from inverter 54 in response to power demand, whichfluctuates rapidly. Also, control of control section 110 and invertercontrol section 111 is not synchronized, and controls inverter 54independently of control section 110 based on the extractable currentvalue Iinv input from control section 110, and on power demand monitorsignal Ms.

Next, referring to FIGS. 9 through 14, we explain the operation of solidoxide fuel cell system 1 according to an embodiment of the invention.FIG. 9 is a control table for setting extractable current value Iinvusing control section 110. FIGS. 10 and 11 are flowcharts fordetermining extractable current value Iinv by applying the control tableshown in FIG. 9.

As shown in FIG. 9, control section 110 increases, decreases, ormaintains extractable current value Iinv based on generating chambertemperature Tfc, generating voltage Vdc output from fuel cell module 2,grid power WI, being the power supplied to facilities such as residencesfrom commercial power sources, interconnect power Winv, being the poweroutput from inverter 54, and fuel supply current value If.

Generating chamber temperature Tfc is the temperature of the generatingchamber 10 housed in individual fuel cell units 16; it is detected bygenerating chamber temperature sensors 142 and input to control section110. Note that in this Specification, temperatures serving as indicatorsof the fuel cell module 2 generating capacity, such as generatingchamber temperature Tfc, are referred to as the “fuel cell moduletemperature.”

Generated voltage Vdc is the output voltage output from fuel cell module2.

Grid power WI is the power supplied by commercial power sources toresidences and the like, which corresponds to total facility powerdemand minus power supplied by fuel cells, and is detected based onpower demand monitor signal Ms.

Interconnect power Winv is the power output from inverter 54. Poweractually extracted at inverter 54 from fuel cell module 2 is detected bypower state detecting sensor 126, and power converted from this power isoutput from inverter 54. The actual extracted current Ic [A] actuallyoutput from fuel cell module 2 is obtained based on the power detectedby power state detecting sensor 126. Therefore power state detectingsensor 126 functions as an extractable current detection device.

Fuel supply current value If is a base current value for obtaining thefuel supply amount, and corresponds to the current value which can begenerated using the fuel supply amount (L/min) supplied to fuel cellmodule 2. Therefore the fuel supply current value If is set so as neverto fall below extractable current value Iinv.

Control section 110 determines whether the current state of fuel cellmodule 2 matches any of line Nos. 1 through 9 in FIG. 9, and changes ormaintains the extractable current value Iinv shown on the right-mostcolumn of FIG. 9.

For example, if all the conditions noted in line No. 1 of FIG. 9 aresimultaneously met, control section 110 changes extractable currentvalue Iinv to reduce it by 5 [mA], as shown in the right column of lineNo. 1. As explained above, in the present embodiment the control cycleof control section 110 is 500 [msec], therefore if the state continuesin which the line No. 1 conditions are met, the extractable currentvalue Iinv is lowered by 5 [mA] every 500 [msec]. In this case theextractable current value Iinv is reduced at a current reduction rate ofchange of 10 [mA/sec].

Similarly, if all the conditions noted in line No. 8 of FIG. 9 aresimultaneously met, control section 110 changes extractable currentvalue Iinv so as to increase it by 10 [mA], as shown in the right handcolumn of line No. 8. Therefore if the state continues in which the lineNo. 8 conditions are met, extractable current value Iinv is raised at afirst current rise rate of 20 [mA/sec].

If none of the conditions in line Nos. 1-8 of FIG. 9 is satisfied, thenthe line No. 9 condition is matched, and extractable current value Iinvis maintained as is without change.

Next, referring to FIGS. 10 and 11, we explain the procedure for judgingthe FIG. 9 control table conditions. Note that letters A-D in FIGS. 10and 11 indicate processing endpoints. For example, the flow transitionfrom “C” in FIG. 10 to “C” in FIG. 11.

As explained below, even under conditions when extractable current valueIinv should be increased, such as when power demand is increasing,control section 110 increases extractable current value Iinv only whennone of the predetermined multiple increase limit conditions is met.Furthermore, the increase limit conditions include multiple currentreducing conditions and current maintenance conditions, and when theseconditions are met, extractable current value Iinv is reduced ormaintained. The multiple current reducing conditions (steps S5, S7, S9,S11, and S13 in FIG. 10) are applied with priority before the multiplecurrent maintenance conditions (steps S15, S16, S17, S18, and S19 inFIG. 11).

First, step S1 in FIG. 10 is a step for judging whether an extremelylarge deviation has occurred between extractable current value Iinv andactual extracted current Ic, whereby a judgment is made as to whether adeviation of greater than 1000 [mA] has occurred between the two. Thecase in which a deviation larger than 1000 [mA] occurs for the firsttime during a control cycle when the difference between extractablecurrent value Iinv and actual extracted current Ic is small is the casein which a sharp reduction in total power demand occurs, or actualextracted current Ic is sharply reduced for some reason, producing adeviation, in which case the system advances to step S2.

In step S2, a judgment is made as to whether grid power WI is less than50 [W]. If grid power WI is less than 50 [W], there is a highprobability of a “reverse current flow (grid power WI turns negative)”occurring, in which output power from inverter 54 flows into thecommercial power supply. Therefore this state is judged to be one inwhich inverter 54 has suddenly reduced actual extracted current Ic inorder to prevent the occurrence of a reverse current flow due to a largedrop in total power demand according to the determinations made in S2and S1. Note that the reason for setting the value of grid power WI inS2 at 50 [W] is to provide a 50 [W] margin so that reverse current flowwill not occur under any circumstance.

Next, if a YES is judged in both S1 and S2, i.e., in cases when ananti-reverse current flow control is performed by inverter 54 inconjunction with a large drop in total power demand, control section 110in step S3 suddenly reduces the value of the extractable current valueIinv instructed to inverter control section 111 down to the value of theactual extracted current Ic (corresponding to FIG. 9, No. 6). With thecompletion of the processing in step S3, one iteration of the FIG. 10and FIG. 11 flowcharts is completed. Inverter 54 extracts actualextracted current Ic in a range not exceeding the value of extractablecurrent Iinv, therefore by reducing the extractable current value Iinvsuch that the extractable current value Iinv=actual extracted currentIc, inverter 54 is restricted from responses such as arbitrarilyincreasing extracted current beyond the current extracted current valueIc. If total power demand suddenly drops, there is a high probabilitythat total power demand will soon after quickly recover (increase), butif inverter 54 suddenly extracts power in order to respond to therecovered total power demand when there is a large deviation exceeding1000 [mA], there can be a control overshoot or the like resulting in theinverter 54 performing a power extraction which mistakenly exceeds powerdemand or extractable current value Iinv; this is prevented in advance.In other words, with a small deviation such as 1000 [mA] or less,inverter 54 is allowed to quickly perform a power extraction up toextractable current value Iinv, which is at a higher level than actualextracted current Ic, since no control is executed to cause theextractable current value Iinv to be the actual extracted current Ic.This is a further measure, taken to enable quick following of therecovery of total power demand, since no problem such as excessive powerextraction due to overshoot arises if the deviation is small.

On the other hand, if a judgment is made in the step S1 and S2determinations that the situation is not one in which a reverse currentassociated with a very large drop in total power demand will arise, thesystem advances to step S4. In step S4 a judgment is made of whetherextractable current value Iinv is greater than 1 A. If extractablecurrent value Iinv is greater than 1 A, the system advances to step S5,and a judgment is made as to whether generating voltage Vdc is less than95 V. If generating voltage Vdc is less than 95 V, the system advancesto step S6.

In step S6, control section 110 reduces the value of the extractablecurrent value Iinv instructed to inverter control section 111 by 10 [mA](corresponding to line No. 4 in FIG. 9). With the completion of the stepS6 processing, one iteration of the FIG. 10 and FIG. 11 flowcharts iscompleted. If the processing in step S6 is continuously executed eachtime the FIG. 10 flowchart is executed, the extractable current valueIinv is decreased at a current decrease change rate of 20 [mA/sec]. Ifgenerating voltage Vdc is less than 95 V, a voltage decrease is assumedto occur due to degradation of the fuel cell module when power isextracted at inverter 54 from fuel cell module 2, therefore by reducingextractable current value Iinv, the current extracted at inverter 54 issuppressed, thereby lightening the load imposed on fuel cell module 2.

Meanwhile, if generating voltage Vdc is 95 V or greater in step S5, thesystem advances to step S7. In step S7 a judgment is made as to whetherinterconnect power Winv exceeds 710 W. If interconnect power Winvexceeds 710 W, the system advances to step S8, and step S8 controlsection 110 reduces the value of the extractable current value Iinvinstructed to inverter control section 111 by 5 [mA] (corresponding toline No. 5 in FIG. 9). In other words, if interconnect power Winvexceeds 710 [W], the output power from fuel cell module 2 exceeds ratedpower, therefore the current extracted from fuel cell module 2 isreduced so as not to exceed rated power. With the completion of theprocessing in step S8, one iteration of the FIG. 10 and FIG. 11flowcharts is completed. If the processing in step S8 is continuouslyexecuted each time the FIG. 10 flowchart is executed, the extractablecurrent value Iinv is decreased at a current decrease change rate of 10[mA/sec].

Thus by using those of the multiple current reduction conditions whichapply, control section 110 changes the extractable current value Iinv sothat the rates of change at which the extractable current value Tiny isreduced differ.

In step S7, meanwhile, if interconnect power Winv is 710 [W] or less,the system advances to step S9. In step S9, a judgment is made as towhether generating chamber temperature Tfc exceeds 850 [° C.]. Ifgenerating chamber temperature Tfc exceeds 850 [° C.], the systemadvances to step S10; in step S10, control section 110 reduces the valueof the extractable current value Iinv instructed to inverter controlsection 111 by 5 [mA] (corresponding to line No. 2 in FIG. 9). I.e., ifgenerating chamber temperature Tfc exceeds 850[° C.], the appropriateoperating temperature for fuel cell module 2 is exceeded, therefore thevalue of extractable current Iinv is reduced and the system waits for adrop in temperature. With the completion of the processing in step S10,one iteration of the FIG. 10 and FIG. 11 flowcharts is completed. If theprocessing in step S10 is continuously executed each time the FIG. 10flowchart is executed, extractable current value Iinv is decreased at acurrent decrease change rate of 10 [mA/sec].

On the other hand, if generating chamber temperature Tfc is 850 [° C.]or less in step S9, the system advances to step S11. In step S11 ajudgment is made as to whether generating chamber temperature Tfc isless than 550[° C.]. If generating chamber temperature Tfc is less than550[° C.], the system advances to step S12; in step S12, control section110 reduces the value of the extractable current value Iinv instructedto inverter control section 111 by 5 [mA] (corresponding to line No. 3in FIG. 9). In other words, if generating chamber temperature Tfc isless than 550[° C.], the temperature is below the appropriatetemperature at which fuel cell module 2 can generate electricity, so thevalue of extractable current Iinv is reduced. Fuel consumed forelectrical generation is thus reduced, and fuel is directed to heatingindividual fuel cell units 16, raising the temperature. With thecompletion of the processing in step S12, one iteration of the FIG. 10and FIG. 11 flowcharts is completed. If the processing in step S12 iscontinuously executed each time the FIG. 10 flowchart is executed,extractable current value Iinv is decreased at a current decrease changerate of 10 [mA/sec].

On the other hand, if generating chamber temperature Tfc is 550[° C.] orgreater in step S11, the system advances to step S13. In step S13 ajudgment is made as to whether the difference between extractablecurrent value Iinv and actual extracted current Ic exceeds 400 [mA] andextractable current value Iinv exceeds 1 A. If the difference betweenthe extractable current value Iinv and actual extracted current Icexceeds 400 [mA] and the extractable current value Iinv exceeds 1 A, thesystem advances to step S14, and in step S14, control section 110reduces the value of the extractable current value Iinv instructed toinverter control section 111 by 5 [mA] (corresponding to line No. 1 inFIG. 9). In other words, if the difference between extractable currentvalue Iinv and actual extracted current Ic exceeds 400 [mA], there istoo little extracted current Ic actually extracted from fuel cell module2 relative to the extractable current value Iinv which can be extracted,and fuel is being wastefully supplied, so the extractable current Tinyis reduced and fuel wastage is suppressed. With the completion of theprocessing in step S14, one iteration of the FIG. 10 and FIG. 11flowcharts is completed. If the processing in step S14 is continuouslyexecuted each time the FIG. 10 flowchart is executed, extractablecurrent value Iinv is decreased at a current decrease change rate of 10[mA/sec].

Thus if even one of the multiple current reduction conditions (steps S5,S7, S9, S11, and S13 in FIG. 10) applies, extractable current value Iinvis reduced even when the power demand is rising (steps S6, S8, S10, S12,S14).

In step S4, meanwhile, when extractable current value Tiny is 1 [A] orless, and in step S13, when the difference between extractable currentvalue Iinv and actual extracted current value Ic is 400 [mA] or less,the system advances to step S15 in FIG. 11.

In step S15, a judgment is made as to whether the difference betweenextractable current value Iinv and actual extracted current value Ic is300 [mA] or less; in step S16 a judgment is made as to whether generatedvoltage Vdc is 100 [V] or greater; in step S17, a judgment is made as towhether extractable current value Iinv is 690 [W] or less; in step S18 ajudgment is made as to whether generating chamber temperature Tfc is600[° C.] or more; and in step S19, a judgment is made as to whethergrid power WI exceeds 40 [W]. If all of these conditions are satisfied,the system advances to step S20; if there is even one which is notsatisfied (corresponding to line No. 9 in FIG. 9), the system advancesto step S21. In step S21, the value of extractable current Iinv is notchanged but maintained at the previous value, and one iteration of theFIG. 10 and FIG. 11 flowcharts is completed.

Thus in the fuel cell system 1 of the present embodiment, if certainconditions are not met, even when power demand is rising, extractablecurrent value Iinv is kept constant (step S21 in FIG. 11). Focusing ongenerating chamber temperature Tfc, when generating chamber temperatureTfc exceeds the upper limit threshold value of 850[° C.], extractablecurrent value Iinv is lowered (steps S9, S10 in FIG. 10), and ifgenerating chamber temperature Tfc is less than the lower limitthreshold value of 600[° C.], the extractable current value Iinv ismaintained (steps S18, S21 in FIG. 11). If generating chambertemperature Tfc is even lower, below 550[° C.], extractable currentvalue Iinv is reduced (steps S11, S12 in FIG. 10).

On the other hand, in the processing which occurs in step S20 andbeyond, the value of extractable current Iinv is increased. Controlsection 110 increases the extractable current value Iinv (steps S22, S23in FIG. 11) only when none of the multiple current maintenanceconditions (steps S15, S16, S17, S18, and S19 in FIG. 11) is matched.

I.e., when the difference between extractable current value Iinv andactual extracted current Ic exceeds 300 [mA] (step S15), that differencebetween extractable current value Iinv and actual extracted current Icis relatively large, therefore the extractable current value Iinv shouldnot be increased. If generating voltage Vdc is lower than 100 V (stepS16), then extractable current value Iinv should not be raised,increasing the current extractable from fuel cell module 2. Furthermore,if interconnect power Winv exceeds 690 [W] (step S17), the output powerfrom fuel cell module 2 has already essentially reached the rated outputpower, therefore the current which can be extracted from fuel cellmodule 2 should not be increased.

In addition, if generating chamber temperature Tfc is less than 600[°C.] (step S18), fuel cell module 2 has not reached a temperature atwhich electricity can be sufficiently generated, therefore theextractable current value Tiny should not be raised, and currentextractable from fuel cell module 2 increased, thereby placing a load onindividual fuel cell units 16. If grid power WI is less than 40 [W](step S19), “reverse power flow” can easily occur, therefore the currentextractable from fuel cell module 2 should not be increased.

If all the conditions from steps S15 through S19 are met, the systemadvances to step S20. In step S20 a judgment is made as to whether thedifference between fuel supply current value If and actual extractedcurrent value Ic is 1000 [mA] or greater. A fuel gas supply amountcorresponding to fuel supply current value If is obtained and suppliedto fuel cell module 2, and the system is generating electricity. Inother words, this value is a conversion of the electrical current valuewhich can be generated by fuel cell module 2 using that fuel. Forexample, if a fuel supply amount [L/min] corresponding to a fuel supplycurrent value If=5 [A] is being supplied, fuel cell module 2 ispotentially capable of safely and stably outputting a 5 [A] current.Therefore if the difference between the fuel supply current value If andthe actual extracted current value Ic is 1000 [mA], this means that anamount of fuel capable of outputting 1 [A] more current than theactually generated extracted current value Ic is being supplied to fuelcell module 2.

If, in step S20, the difference between fuel supply current value If andactual extracted current value Ic is 1000 [mA] or greater, the systemadvances to step S22; if less than 1000 [mA], the system advances tostep S23. In step S22, because a large amount of extra fuel is beingsupplied to fuel cell module 2, control section 110 increases the valueof the extractable current value Iinv instructed to the inverter controlsection 111 by 100 [mA] (corresponding to line No. 9 in FIG. 9), rapidlyraising the extractable current value Iinv. The completion of the stepS22 processing completes one iteration of the FIGS. 10 and 11flowcharts. If the processing in step S22 is continuously executed eachtime the FIG. 11 flowchart is executed, extractable current value Iinvis raised at a second current increase rate of change, being 200[mA/sec].

On the other hand, the conditions for raising extractable current valueIinv are present in step S23, but since this is not a situation in whicha large amount of extra fuel is being supplied to fuel cell module 2,control section 110 increases the value of extractable current Iinvinstructed to inverter control section 111 by 10 [mA] (corresponding toline No. 8 in FIG. 9), gradually raising the extractable current valueIinv. With the completion of the processing in step S23, one iterationof the FIG. 10 and FIG. 11 flowcharts is completed. If the processing instep S23 is continuously executed each time the FIG. 11 flow chart isexecuted, extractable current value Iinv will be increased at a firstcurrent increase change rate of 20 [mA/sec].

Next, referring to FIGS. 12 through 14, we explain one example of theoperation of solid oxide fuel cell system 1 according to the presentembodiment.

FIG. 12 shows a graph of power on the top and current on the bottom.

First, as shown by the thin solid line on the top half of FIG. 12, ifthe total power demand from facilities such as residence 200 etc. isgradually increasing as it fluctuates, extractable current value Iinvwill also be gradually increased in response (times t0-t1 in FIG. 12).During this interval, step S23 in the FIG. 11 flowchart is repeated, andextractable current value Iinv is raised at a first current rise rate of20 [mΛ/sec]. If extractable current value Iinv is input from controlsection 110, inverter control section 111 controls 54 independently fromthe control of control section 110, and an actual extracted current Icnot exceeding extractable current value Iinv is extracted from fuel cellmodule 2 At times t0-t1 in FIG. 12, the total power demand at all timesexceeds extractable current value Iinv, therefore actual extractedcurrent Ic matches extractable current value Iinv, which is the upperlimit value of what can be extracted, and interconnect power Winv isalso raised together with actual extracted current Ic. In suchcircumstances, notwithstanding the fact that inverter control section111 is controlling inverter 54 independently from control section 110,actual extracted current Ic is under the dominance of control section110. Note that fuel supply current value If is raised slightly ahead ofextractable current value Iinv. The insufficient power resulting fromthe difference between the total power demand and interconnect powerWinv is made up by grid power WI.

Next, at time t1 in FIG. 12, if total power demand suddenly decreases,inverter control section 111 controls inverter 54 in response to thisreduction, causing a decrease in the actual extracted current Ic(interconnect power Winv) extracted from fuel cell module 2. Controlsection 110 causes extractable current value Iinv to be reduced to thesame value as actual extracted current Ic (step S3 in FIG. 10) so thatno excessive power extraction occurs due to the previously notedovershoot. At the same time, at time t1, control section 110 maintainsthe value of fuel supply current If at the previous value, not reducingit. This is because suddenly reducing the fuel supply amount (fuelsupply current value If) together with extractable current value Iinvinvites a sudden temperature drop, so that not only does the operationof fuel cell module 2 become unstable, and since there is a highprobability that the power demand will increase immediately after asudden drop in power demand, one would like to be able follow thisquickly, but because of the reduction in the temperature of fuel cellmodule 2, a long time period is required for recovery, so the fuelsupply current value If is not reduced. Therefore if extractable currentvalue Iinv is suddenly reduced, fuel supply current value If is reducedat a further delay. In a state whereby fuel supply current value If isbeing maintained immediately after the reduction in the extractablecurrent value Iinv, there is a margin in the fuel supply amount to thefuel cell module 2 relative to extractable current value Iinv.

At time t1-t2 in FIG. 12, the total power demand is still reduced,therefore generating chamber 10 maintains a fixed extractable currentvalue Iinv (step S21 in FIG. 11), and the fuel supply current value Ifis also maintained at a fixed value, with extra fuel.

Next, when total power demand again rises at time t2, because fuelsupply current value If is maintained at a fixed level and there isextra margin in the fuel supply amount to fuel cell module 2, controlsection 110 causes the value of the extractable current value Iinv torapidly rise at a second current rise rate of change which is 200[mA/sec] greater than the normal rate of change (the first current riserate of change) (step S22 in FIG. 11). This results in improved loadfollowing characteristics relative to total power demand. I.e., when thevalue of extractable current Iinv output from control section 110 israised, inverter control section 111 extracts actual extracted currentIc from fuel cell module 2 within the range of this increasedextractable current value Tiny. Rapidly raising the value of extractablecurrent Iinv enables the extraction of a large power from fuel cellmodule 2 suited to fuel supply current value If, so that the amount ofgrid power WI used can be suppressed.

Note that in the present embodiment the extractable current value Iinvis raised at a 200 [mA/sec] large current rise rate of change, which isthe second current rise rate of change, and at a 20 [mA/sec] normalcurrent rise rate of change, which is the first current rise rate ofchange. If the current rise rate of change is the first current riserate of change only, extractable current value Iinv rises gradually fromtime t2, as shown by the double dot and dash line in FIG. 12. Thereforeif the current rise rate of change is not raised rapidly, then even ifthe fuel supply amounts are the same, actual extracted current Ic isreduced by only the amount of the diagonally shaded region in FIG. 12,and fuel is wastefully expended. Conversely, the problem of slowtemperature change, which is a major issue for solid oxide fuel cellsresulting from the use of power demand prediction control in thistechnology, is solved, and load following characteristics can be rapidlyraised.

Note that in this embodiment the fuel supply current value If ismaintained as is, but if there is too great a deviation between fuelsupply current value If and extractable current value Iinv, the largeover-increase in the amount of extra fuel will be wasteful evenconsidering the re-restoration of total power demand, so it can saidthat an even more preferable response is to maintain the fuel supplycurrent value while keeping the deviation amount from becoming toolarge.

At time t3 in FIG. 12, when the difference between fuel supply currentvalue If and actual extracted current Ic is reduced, control section 110changes the current rise rate of change to 20 [mA/sec], which is thefirst current rise rate of change, making the rise in extractablecurrent value Iinv gradual (step S23 in FIG. 11). This is to prevent theoccurrence of fuel depletion caused by the operational offset withinverter control section 111, which controls inverter 54 independentlyof control section 110 when extractable current value Iinv is suddenlyraised in a state in which there is little margin in the fuel supplyamount (the fuel supply current value If).

Next, when the value of extractable current Iinv has risen and the fuelsupply current value If is approached at time t4 in FIG. 12, the valueof fuel supply current value If is also raised together with extractablecurrent value Iinv, so a certain reserve amount is secured relative toextractable current value Iinv.

Next, referring to FIG. 13, we explain another example of the operationof solid oxide fuel cell system 1 according to the present embodiment.

In the operational example shown in FIG. 12, after total power demanddrops at time t1, total power demand shifts to rising during the periodwhen the fuel supply current value If is being maintained. Relative tothis, in the example shown in FIG. 13, the time after total power demanddrops until this shift to rising is lengthy. Since supplied fuel iswasted when the time during which a large fuel supply current value Ifis maintained, fuel supply current value If is reduced after apredetermined fuel reduction standby time tw elapses.

After the sharp drop in total power demand at time t11 in FIG. 13, totalpower demand stays low until time t13. In the present embodiment,control section 110 is constituted so that if total power demand dropssharply and extractable current value Iinv is reduced to actualextracted current Ic, the fuel supply current value If is maintained ata fixed level thereafter during the interval of the 15 sec fuelreduction standby time tw.

In the example shown in FIG. 13, after the sharp drop in total powerdemand at time t11, that total power demand stays low even at time t12when the fuel reduction standby time tw has elapsed, therefore controlsection 110 reduces the fuel supply current value If (the fuel supplyamount) at a predetermined reduction rate of change starting at timet12. A value is selected for this reduction rate of change such thatfuel cell module 2 can maintain appropriate operation. Note that in theexample shown in FIG. 13, fuel supply current value If is reduced afterthe elapse of fuel reduction standby time tw, but if the differencebetween extractable current value Iinv and fuel supply current value Ifis less than a certain amount, fuel supply current value If ismaintained without being reduced, notwithstanding the elapse of fuelreduction standby time tw. Thus minute fluctuations in fuel supplycurrent value If can be prevented from adversely affecting the operationof fuel cell module 2.

Next, when total power demand rises at time t13 in FIG. 13, controlsection 110 causes extractable current value Iinv and fuel supplycurrent value If to rise. However, the rise in extractable current valueIinv at this point does not occur immediately after the sharp reductionin extractable current value Iinv; the fuel supply current value If isalso reduced, so the rate of change in the extractable current valueIinv current rise is set at the normal value, which is the first currentrise rate of change of 20 [mA/sec]. I.e., at time t13 in FIG. 13, theconditions of step S20 in FIG. 11 are not met, and step S23 is executed.

Note that the present embodiment is constituted so that after reducingthe fuel supply current value If for 15 seconds, maintenance of theexcess fuel level is stopped, but it can be said that in cases wheretotal power demand declines even further, the probability that totalpower demand will recover and rise is low, therefore rapidly reducingfuel supply current value If without waiting the 15 seconds to maintainfuel supply current value If is even more preferable.

Next, referring to FIG. 14, we explain another example of the operationof solid oxide fuel cell system 1 according to the present embodiment.

In the operational example shown in FIG. 14, after a sharp drop in totalpower demand, that total power demand again sharply drops, withoutrising. In such cases the probability that total power demand willquickly shift to rising is assumed to be low, even if fuel reductionstandby time tw has elapsed after the first sharp drop, thereforecontrol section 110 reduces fuel supply current value If.

In the example shown in FIG. 14, after a sharp drop in total powerdemand at time t21, at time t22 before the elapse of the 15 sec fuelreduction standby time tw, total power demand again drops sharply. Thusif the difference between fuel supply current value If and extractablecurrent value Iinv expands, control section 110 reduces the fuel supplycurrent value If (the fuel supply amount) at a predetermined reductionrate of change starting at time t22, even if fuel reduction standby timetw has not yet elapsed since the first sharp reduction in total powerdemand. Next, when total power demand rises at time t23 in FIG. 14,extractable current value Iinv and fuel supply current value If areincreased.

In the solid oxide fuel cell system 1 of the embodiment according to thepresent invention, in circumstances corresponding to certain increaselimit conditions (steps S5, S7, S9, S11, and S13 in FIG. 10 and stepsS15, S16, S17, S18, and S19 in FIG. 11), even if total power demand isrising, control section 110 maintains extractable current value Iinvwithout causing it to rise (step S21 in FIG. 11) or reduces extractablecurrent value Iinv (steps S6, S8, S10, S12, and S14 in FIG. 10),therefore damage to fuel cell module 2 by fuel depletion or the like canbe reliably avoided and fuel cell module 2 can be stably operated. Also,even when total power demand is rising, the condition of fuel cellmodule 2 can be quickly restored by maintaining or reducing extractablecurrent value Iinv, thereby actually increasing generating efficiencycompared to the option of immediately raising extractable current valueIinv.

In general, inverter 54 is controlled with high response characteristicsso that the requisite current can be extracted from fuel cell module 2in response to sudden changes in power demand. Meanwhile if the fuelsupply amount supplied to fuel cell module 2 is suddenly changed,electrical generation by fuel cell module 2 can become unstable so thathighly responsive control cannot be achieved. Moreover, multiple factorssuch as temperature affect the generating capacity of fuel cell module2, such that the generating capacity of fuel cell module 2 does notnecessarily rise immediately when the fuel supply amount is increased.In some cases, therefore, ignoring the status of the fuel cell moduleand simply increasing extracted current while increasing the fuel supplyamount, as in conventional fuel cells, can impose an excessive load onthe fuel cell module, hastening the degradation of the fuel cell module.

Also, in the solid oxide fuel cell system 1 of the embodiment accordingto the present invention, if the value of parameter such as generatingchamber temperature Tfc is inappropriate for increasing extractablecurrent value Iinv, then the value of extractable current Iinv ismaintained (steps S18 and S21 in FIG. 11), so the temperature of theslow-response fuel cell module 2 rises without imposing a load on fuelcell module 2, and a determination can be made as to whether or not itsstatus is headed toward recovery. If the fuel cell module 2 temperaturedrops further and conditions further degrade, extractable current valueIinv can be reduced (steps S11 and S12 in FIG. 10) to actively lightenthe load on fuel cell module 2.

In addition, in the solid oxide fuel cell system 1 of the presentembodiment, current reduction conditions (steps S5, S7, S9, S11, S13 inFIG. 10) are applied with priority over current maintenance conditions(steps S15, S16, S17, S18, S19 in FIG. 11), so in cases where there is apossibility that a continued load on fuel cell module 2 will lead todamage to fuel cell module 2, that load can be quickly lightened so thatdamage to fuel cell module 2 is reliably prevented.

Also, using the solid oxide fuel cell system 1 of the presentembodiment, increases in the extractable current value (steps S22, S23in FIG. 11) which impose a large load on fuel cell module 2 are executedonly when there is not a match with all of the current maintenanceconditions (steps S15, S16, S17, S18, and S19 in FIG. 11), therefore incases when the state of fuel cell module 2 worsens more as theextractable current value Iinv is increased, the extractable currentvalue Iinv is maintained at a fixed value. If extractable current valueIinv matches even one of the current reduction conditions (steps S5, S7,S9, S11, or S13 in FIG. 10), it is immediately reduced (steps S6, S8,S10, S12, S14 in FIG. 10), and damage to the fuel cell module 2 can bereliably prevented.

Furthermore, in the solid oxide fuel cell system 1 of the presentembodiment, for conditions requiring a rapid reduction of the load onfuel cell module 2, extractable current value Iinv can be rapidlyreduced (step S6 in FIG. 10) to quickly reduce load, whereas in caseswhere a rapid reduction in load is not required, a gradual reduction inthe extractable current value (steps S8, S10, S12, S14 in FIG. 10) canbe used to raise extractable current value Tiny when the condition offuel cell module 2 has improved, thereby saving the time needed untilthe necessary value is restored.

Using the solid oxide fuel cell system 1 of the present embodiment,increase limit conditions are judged based on the fuel cell module 2temperature, the fuel cell module output voltage Vdc, actual extractedcurrent Ic, and extractable current value Iinv, therefore by parallelmonitoring of multiple parameters having differing effects on thegenerating capacity of fuel cell module 2, the generating capacity offuel cell module 2 can be rapidly restored while suppressing degradationof fuel cell module 2.

Furthermore, with the solid oxide fuel cell system 1 of the presentembodiment, in cases where the fuel cell module 2 temperature hasdropped below the lower limit threshold (step S18 in FIG. 11), theextractable current value Tiny can be maintained (step S21 in FIG. 11)and recovery of the temperature awaited, and if further temperaturedrops occur (step S11 in FIG. 10), the time required until restorationof the requisite extractable current value Iinv can be shortened. On therising side of the fuel cell module 2 temperature, damage to fuel cellmodule 2 can be prevented by immediately reducing the extractablecurrent value (steps S9, S10 in FIG. 10).

EXPLANATION OF REFERENCE NUMERALS

-   -   1: solid oxide fuel cell system    -   2: fuel cell module    -   4: auxiliary unit    -   8: sealed space    -   10: electrical generating chamber    -   12: fuel cell assembly    -   14: fuel cell stack    -   16: individual fuel cell units (solid oxide fuel cells)    -   18: combustion chamber    -   20: reformer    -   22: heat exchanger for air    -   24: water supply source    -   26: pure water tank    -   28: water flow regulator unit (water supply device)    -   30: fuel supply source    -   38: fuel flow regulator unit (fuel supply device)    -   40: air supply source    -   44: reforming air flow regulator unit    -   45: generating air flow regulator unit    -   46: first heater    -   48: second heater    -   50: hot water production device    -   52: control box    -   54: inverter    -   83: ignition device    -   84: fuel cells    -   110: control section (fuel cell controller)    -   110 a: first power demand detection device    -   111: inverter control section (inverter controller)    -   111 a: second power demand detection device    -   112: operating device    -   114: display device    -   116: warning device    -   126: power state detecting sensor (extractable current detection        device)    -   132: fuel flow volume sensor (fuel supply amount detection        sensor)    -   138: pressure sensor (reformer pressure sensor)    -   142: generating chamber temperature sensor (temperature        detection means)    -   150: outside air temperature sensor    -   200: residence    -   202: fuel cell    -   204: grid power    -   206: current transformer    -   208: fuel cell module    -   210: filter    -   212: inverter

1. A solid oxide fuel cell system for generating variable power in response to power demand, comprising: a fuel cell module that generates electricity using supplied fuel; a fuel supply device that supplies fuel to the fuel cell module; a power demand detection device that detects power demand; a controller that controls the amount of fuel supplied by the fuel supply device based on the power demand detected by the power demand detection device, and that sets an extractable current value, being the maximum current value which can be extracted from the fuel cell module in accordance with the condition of the fuel cell module; an inverter that converts current from the fuel cell module to alternating current within a range not exceeding the extractable current value; and an extractable current detection device that detects the actual extracted current actually extracted from the fuel cell module at the inverter; wherein if predetermined increase-limiting condition is matched, then even when power demand is rising, the controller maintains the constant extractable current value, or lowers the extractable current value, and does not increase the extractable current value.
 2. The solid oxide fuel cell system according to claim 1, wherein the controller is constituted to judge the increase-limiting condition based on certain parameter, such that even when power demand is rising, if predetermined parameter exceeds certain threshold, extractable power is maintained at a fixed value, and if the excess amount over predetermined threshold increases still further, extractable power is reduced.
 3. The solid oxide fuel cell system according to claim 2, wherein the increase-limiting condition includes a current maintenance condition for maintaining extractable currents at a fixed value, and a current reducing condition for reducing extractable current, and the controller applies the current reducing condition with priority over the current maintenance condition.
 4. The solid oxide fuel cell system according to claim 3, wherein the controller is provided with multiple current maintenance conditions and current reducing conditions, respectively, and the extractable current value is increased when none of multiple current maintenance conditions is met, and is reduced when even one of the multiple current reducing conditions is met.
 5. The solid oxide fuel cell system according to claim 4, wherein the controller changes the extractable current value so that the rate of change at which the extractable current value is reduced varies according to which of the multiple current reducing conditions was met.
 6. The solid oxide fuel cell system according to claim 5, wherein increase-limiting condition is judged based multiple parameters selected from among fuel cell module temperature, fuel cell module output voltage, actual extracted current, and extractable current value.
 7. The solid oxide fuel cell system according to claim 6, wherein the temperature of the fuel cell module is judged based on a lower limit threshold value and an upper limit threshold value; if the temperature of the fuel cell module drops below the lower limit threshold value, the extractable current value is maintained at a fixed value; if the fuel cell module temperature drops still further, the extractable current value is lowered; if the fuel cell module temperature rises past the upper limit threshold value, the extractable current value is reduced. 